This would break inputs all over the web. Check out yahoo.com - type into the search box. That auto-suggest feature wouldn’t work based on your proposal.
Use the arrow keys?
Firefox translations are local and open source.
https://searchfox.org/mozilla-central/source/toolkit/components/translations may be a good place to start to learn. https://firefox-source-docs.mozilla.org/toolkit/components/translations/index.html may also be helpful.
We are not an official Mozilla community.
Have you seen my list of migrating communities?
You can grab a profile of performance issues and report them: https://profiler.firefox.com/docs/#/./guide-profiling-firefox-android